1. Field of the Invention
Embodiments described herein generally relate to an apparatus and method for improving the performance of one or more fiber optic sensors. More particularly, embodiments described herein relate to a coating on a fiber configured to improve the performance of the sensor. More particularly still, embodiments described herein relate to a highly thermally conductive material which encapsulates the fiber in order to reduce thermal errors in an interferometric fiber optic gyroscope (IFOG).
2. Description of the Related Art
Optical sensor systems operate by exposing a portion of an optical waveguide to an environmental condition that modulates a light signal transmitted within the optical waveguide. This modulation alters one or more parameters of the light transmitted within the optical waveguide, such as amplitude, power distribution versus frequency/wavelength, phase, or polarization. Analyzing modulated light emerging from the waveguide enables determining values indicative of the environmental condition. Such systems utilize sensors based on, for example, Bragg gratings or interferometers to measure a wide variety of parameters, such as strain, displacement, velocity, acceleration, flow, corrosion, chemical composition, temperature, or pressure. In one example of an optical sensor system, an interferometric fiber optic gyroscope (IFOG) enables measuring angular rotation as it alters the path length of counter-propagating waves of light traveling through a sensing coil of an optical fiber, thereby producing phase changes from which measurements can be made.
Typical IFOG systems use a beam splitter, or coupler, to split light from a light source into counter propagating waves traveling in the sensing coil. A detector having associated electronics measures the phase relationship between the two interfering counter-propagating beams of light that emerge from the opposite ends of the sensing coil. The difference between the phase shifts experienced by the two beams is proportional to the rate of rotation of the platform to which the instrument is fixed, due to the Sagnac effect.
Typical IFOG systems are highly sensitive to changes in the thermal condition around the IFOG. Changes in the temperature surrounding the coiled sensor produces thermal gradients acting across the IFOG sensing coil that result in variant localized thermal expansion of the fiber that produces non-reciprocal phase errors. The result is the Shupe effect which causes sensor drift over time that is both time and temperature dependent. The accuracy of the IFOG is then limited by the Shupe effect. Thermally induced phase errors occur if there is a time-dependent temperature gradient along the fiber. Non-reciprocity phase errors arise when clockwise and counter clockwise counter rotating beams traverse the same region of the fiber at different times. If the fiber's propagation velocity varies at different points along the fiber, the two beams traverse slightly different effective path lengths. The resulting phase shift is indistinguishable from the phase shift caused by rotation. It is very difficult to maintain temperature uniformity of the sensing coil required to eliminate these effects and maintain IFOG accuracy—even under steady thermal operating conditions.
Currently, one method for reducing the Shupe effect is through complex winding patterns. The fiber in the sensing coil is wound so that the sections of the fiber that are at equal distance from the coil center are beside each other, such as in the quadra, hexa, or octapolar wind. The complex winding allows the local thermal effects for each section of the fiber to be experienced at the same moment and at the same magnitude for each of the counter rotating beams. These complex winding patterns are difficult to assemble. Further, even with extreme care in winding, the sensing coil exhibits residual drift. The residual drift is due to an incomplete cancellation of the different contributions on a complex and nonlinear temperature model based upon temperature and time derivatives. Although the temperature model is well understood, precise thermal monitoring of the sensing coil required to compensate for Shupe effect errors, is difficult to implement due to the low thermal mass, insulating properties of typical polymer-coated glass optical fibers. The complex winding patterns minimize gross thermally induced Shupe effect errors. However, inherent winding imperfections and thermal transients lead to residual drift over time and become a function of a complex nonlinear thermal model.
Therefore, a need exists for a method and apparatus for improved thermal performance in Sagnac fiber optic sensors. Moreover, a need exists for an optical fiber coating system and monitoring system to improve the thermal performance in Sagnac fiber optic sensors.